It is known that artificial illumination devices are currently available for indoor environments, designed to improve the visual comfort of the users. In particular, illumination devices are known which simulate natural illumination, i.e. the type of illumination present in outdoor environments. The well-known characteristics of natural illumination depend on the interaction between the light rays produced by the sun and the earth's atmosphere.
For example, the European patent application EP2304480 describes an illumination device which comprises a light source, apt to generate visible light, and a panel containing nanoparticles. In use, the panel receives the light rays coming from the light source and acts as a so-called Rayleigh diffuser, i.e. it scatters the light rays analogously to what occurs in the earth's atmosphere in clear sky conditions. Further details relative to the panel as per the European patent application EP2304480 are described in the European patent application EP2304478.
In particular, the illumination device described in the European patent application EP2304480 simulates natural illumination as it generates, within an environment illuminated by it, direct light having low correlated colour temperature (CCT), which simulates the direct light coming from the sun and generates shadows, in the presence of illuminated objects; furthermore, the illumination device generates diffuse light with high CCT, which simulates the light of the sky and gives the shadows a blue tone.
The patent application PCT/IB2013/060141, filed on 14 Nov. 2013, describes an illumination device based on the fact that, given a light source which stands out against a background and is observed by an observer through a Rayleigh diffuser panel, the observer has difficulty in perceiving the actual distance of the light source, if i) said distance is greater than a limit value, ii) the source is sufficiently intense to make focusing difficult by the observer and iii) the background is uniform (and, preferably, black). In practice, the illumination device is based on the fact that the observer is induced to perceive the light emitted by the diffuser panel as coming from a virtually infinite distance, provided that the light generated by the light source is within the observer's field of vision. In fact, the diffuser panel acts as a secondary light radiation source which, due to the high spatial uniformity, prevents the observer from evaluating the actual distance that separates him/her from the diffuser panel.
The illumination system described in the patent application PCT/IB2013/060141 allows very credible simulation of natural illumination conditions, and in particular allows the generation of visible optical stimuli comparable to those generated when a room is illuminated by a window. However, this illumination device is limited to artificially replicating stimuli of a mainly visible nature whereas, in reality, when an observer is in a room illuminated by a window, stimuli of different types can contribute to the overall perception of the observer.